Alkaline earth metal nitride represented by M3N2 (M represents Ca, Sr or Ba) is used for a raw material of aluminum nitride which is used in a semiconductor device, ceramic particles for a metal sliding member, a material for forming a battery electrode, conductive fine particles, and the like (Patent Document 1). Patent Document 1 discloses a method of producing M3N2 (M represents Be, Mg, Ca, Sr or Ba), M2N (M represents Be, Mg, Ca, Sr or Ba) or the like, wherein a Group II metal amide is thermally decomposed, and Patent Document 2 discloses a method of producing high-purity M3N2 (M represents Be, Mg, Ca, Sr or Ba), wherein ammonia is reacted with a Group II metal to be liquefied, and the obtained Group II metal amide is thermally decomposed.
As a calcium nitride, α-Ca3N2, β-Ca3N2, γ-Ca3N2, Ca11N8, Ca2N and the like are known. In addition, Ca2NH, CaNH, Ca (NH2)2 and the like (hereinafter, may be referred as a “Ca—N—H-based compound” in some cases), which are hydrides of the calcium nitride are also known.
Ca3N2 is obtained by dissolving Ca in liquid ammonia and performing thermal decomposition under a nitrogen atmosphere. As shown in the following formula, Ca3N2 reacts with moisture in air to generate calcium hydroxide and ammonia. This reaction also occurs in water.Ca3N2+6H2O→3Ca(OH)2+2NH3 
In addition, it is known that Ca2N is easily oxidized, and is a very unstable material. It has been informed that Ca2N can stably exist in Ar in a range which is equal to or less than 1000° C., or in nitrogen in a range which is between 250° C. to 800° C. (Non Patent Document 1).
On the other hand, in 2013, the present inventors found that Ca2N is a two-dimensional electride (Non Patent Document 2). Ca2N:e− is a layered compound in which electrons are linked with each other as an anion between layers formed by [Ca2N]+, and is obtained by heating Ca3N2 and metal Ca in a vacuum. It has been reported that a conductive electron concentration of Ca2N:e− is 1.39×1022/cm3, and a work function thereof is 2.6 eV. Thereafter, the two-dimensional electride is reported in Non Patent Document 3.
Further, the application for a patent has been filed by the present inventors with respect to an invention relating to a nitride electride which consists of a nitride, wherein the nitride has a layered crystal structure and is represented by ion formula [AE2N]+e− (AE represents at least one element selected from Ca, Sr and Ba) (Patent Document 3).
On the other hand, as a synthesis of ammonia in which a gas containing hydrogen and nitrogen is used as a raw material, a Haber-Bosch method which uses an iron oxide as a catalyst and uses alumina or potassium oxide as a promoter has been widely used, and the synthesis method has not been significantly changed for the past 100 years. The synthesis of ammonia according to the Haber-Bosch method is performed by reacting nitrogen and hydrogen gases on the catalyst under a high temperature and a high pressure, which are in a range of 300° C. to 500° C. and in a range of 20 to 40 MPa.
As a catalyst used for synthesizing ammonia, a ternary nitride of a Group VIII metal and a Group VIB metal (Patent Document 4), a cobalt-molybdenum composite nitride (Patent Document 5) and the like are known. The present inventors have developed a supported metal catalyst in which a mayenite type compound is used as a support of an ammonia synthesis catalyst (Patent Documents 6 and 7, and Non Patent Documents 4 and 5).